1. Field of the Invention
The present invention relates to an apparatus for generating raw material gas used in apparatus for growing thin film, and this apparatus for forming a film is used for manufacturing a compound semiconductor device such as a semiconductor laser, a high electron mobility transistor (HEMT) and a Metal-Semiconductor FET (MESFET).
2. Description of the Related Art
Hydride gas such as arsine and phosphine has been used as a material of a V group element in a process of growing a compound semiconductor layer. Since these gases are sealed under a high pressure, they are in danger of explosion, and there is a problem in point of a safety measure because of highly poisonous character thereof. Thus, it is not easy to handle those gases.
Therefore, a liquid organic V group raw material which is in a liquid state at room temperature and has low toxicity is attracting public attention. A method of bubbling a liquid organic V group raw material by gasified hydrogen is generally adopted in order to gasify the liquid organic V group raw material.
A structure of an apparatus for growing a compound semiconductor film in which such a method is adopted is shown in FIG. 1.
In FIG. 1, a reference numeral 1 represents a main hydrogen supply pipe, 2a to 2c represent diluting hydrogen supply pipes, 3a and 3c represent bubbling hydrogen supply pipes, 4a to 4g represent mass flow controllers, 5 represents a valve for saving gas flow or for switching a flow passage, 6A-6C represents a thermostatic tank, 7a represents a first cylinder for containing liquefied tertial butyl phosphina (hereinafter referred to as TBP), 7b represents a second cylinder for containing liquefied trimethyl indium (hereinafter referred to as TMIn), 7c represents a third cylinder for containing liquefied triethyl gallium (hereinafter referred to as TEGa), 8 represents a barrel type reaction chamber, 9 represents a substrate on which a film is formed, 10 represents a pressure gauge, 11 represents a pressure control valve for regulating-the pressure of the reaction chamber 8, 12 represents an exhaust pump, 13 represents a scruber and 14 represents an exhaust pipe.
Next, the operation for growing an InGaP film on the substrate 9 using this apparatus will be described. Besides, the items (1) to (4) described hereunder are operated in parallel with one another.
(1) First supply system of organic metal raw material gas
The flow rate of bubbling hydrogen supplied from the bubbling hydrogen supply pipe 3a is controlled by the mass flow controller 4c, and the controlled bubbling hydrogen is discharged into the TBP liquid in the first cylinder 7a, thereby to perform bubbling, and the TBP gas thus obtained is supplied into the barrel type reaction chamber 8. The TBP liquid is maintained at constant temperature by the thermostatic tank 6A.
Further, the flow rate of diluting hydrogen supplied from the diluting hydrogen supply pipe 2a is controlled by a mass flow controller 4b, and the controlled diluting hydrogen is supplied to a flow passage of TBP gas and introduced into the barrel type reaction chamber 8.
(2) Second supply system of organic metal raw material
The flow rate of bubbling hydrogen supplied through the bubbling hydrogen supply pipe 3b is controlled by the mass flow controller 4e, and the controlled bubbling hydrogen is discharged into the TMIn liquid in the second cylinder 7b thereby to perform bubbling, and the TMIn gas thus obtained is supplied into the barrel type reaction chamber 8. The TMIn liquid is maintained at constant temperature by the thermostatic tank 6b.
Further, the flow rate of diluting hydrogen supplied through the diluting hydrogen supply pipe 2b is controlled by the mass flow controller 4e, and the controlled diluting hydrogen is supplied to a flow passage of TMIn gas and introduced into the barrel type reaction chamber 8.
(3) Third supply system of organic metal raw material
The flow rate of bubbling hydrogen supplied through the bubbling hydrogen supply pipe 3c is controlled by the mass flow controller 4g, and controlled bubbling hydrogen is discharged into a TEGa liquid in the third cylinder 7c thereby to perform bubbling, and the TEGa gas is supplied into the barrel type reaction chamber 8 with hydrogen as carry gas. The TEGa liquid is maintained at constant temperature by the thermostatic tank 6c.
Further, the flow rate of diluting hydrogen supplied through the diluting hydrogen supply pipe 2c is controlled by the mass flow controller 4f, and controlled diluting hydrogen is supplied to the flow passage of TEGa gas and introduced into the barrel type reaction chamber 8.
(4) Supply system of hydrogen only
Hydrogen supplied through the main hydrogen supply pipe 1 and the flow rate of which is controlled by the mass flow controller 4a is supplied into the barrel type reaction chamber 8 having the pressure gauge 10.
In respective gas supply systems described above, valves 5 for saving gas flow and switching the flow passage are provided at important positions of the gas pipes serving as flow passages of gases such as hydrogen, TBP, TMIn and TEGa.
With this, an InGaP film is grown on the substrate 9 installed in the barrel type reaction chamber 8.
The gas exhausted from the barrel type reaction chamber 8 by the exhaust pump 12 is made harmless by the scrubber 13 and exhausted outside through the exhaust pipe 14.
Now, when an InGaP film is grown by using the above-described apparatus for growing a compound semiconductor, the TBP liquid in the first cylinder 7a for TBP is maintained at 10.degree. C. by the thermostatic tank 6a. When the flow rate of bubbling hydrogen discharged therein is set to 500 cc/min, the rate of the group V element to the group III element (hereinafter referred to a V/III ratio) in the reaction chamber 8 reaches only to approximately 10, and a partial pressure of phosphorus (P) sufficient for growing an InGaP film having good film quality is unobtainable. Under such a condition, evenness of the InGaP film on the substrate 9 is decreased, and the surface thereof gets slightly opaque.
Further, in order not to lower the growth speed of the InGaP layer, the V/III ratio in the reaction chamber 8 has to be made 100 in the reaction chamber 8. It is conceivable to connect about 10 pieces of the first cylinders 7a for TBP in parallel as means for achieving the foregoing. Namely, such a method that a plurality of the first cylinders 7a are prepared, bubbling is performed in respective cylinders 7a, and the TBP gas is supplied in parallel at the same time from respective cylinders 7a may be adopted, however, the cost is increased remarkably and the apparatus becomes large in size according to this method.
Since the takeoff quantity of the organic group V raw material gas is small in the bubbling unit such as described above, only one mass flow controller can be provided in one bubbler which contains the liquid organic group V raw material. Therefore, a plurality of bubblers have to be installed in case those layers that require different compositions are continuous.
Further, in the bubbling method, the takeoff quantity of the raw material is unstable, and moreover, it takes a long period of time until the flow rate is stabilized after the flow rate is changed.
Instability of the takeoff quantity by bubbling is shown in FIG. 2. FIG. 2 shows a state that the takeoff quantity of TBP varies when bubbling is performed by supplying bubbling hydrogen at a flow rate of 200 cc/min into the TBP liquid. The axis of abscissas shows elapsed time of film growth, and the axis of ordinates shows an actual flow rate of TBP.
According to FIG. 2, it is realized that the actual flow rate of TBP fluctuates fractionally irrespective of the elapsed time of growth. Such fluctuation deteriorates the reproducibility of a growing semiconductor film, and more specifically the reproducibility of a condition for forming lattice match of a semiconductor layer and so forth.
Saturability of the takeoff quantity of TBP gas by bubbling is shown in FIG. 3. The axis of abscissas shows a set flow rate of TBP gas intended to obtain an increased supply quantity of bubbling hydrogen, and the axis of ordinates shows an actual flow rate which is practically obtainable from the first cylinder 7a.
When the TBP liquid is maintained at a temperature of 10.degree. C., curves of the set flow rate and the actual flow rate show that the set flow rate agrees with the actual flow rate while the set flow rate is small. When the curve is saturated with the increase of the set flow rate and reaches to 400 cc/min or higher, however, the actual flow rate proportional to the supply quantity of the bubbling hydrogen becomes unobtainable.
The reasons for the above are that the quantity gasified by bubbling stops increasing in proportion to the discharge quantity of hydrogen because bubbles are joined together or become larger in size when the discharge quantity of bubbling hydrogen increases to a certain quantity, and further that the quantity of TBP which can be taken out as gas becomes non-proportional to the discharge quantity of hydrogen gas due to the relationship with a saturated vapor pressure. With this, the limit of the bubbling principle is shown.
Because of the fact that the decomposition temperature of the group V raw material is high and the vapor pressure of the group V element is high, it is required to supply the group V element excessively than a stoichiometric value of the grown film to the reaction chamber. Particularly, when a thin film having a large area is grown, it is required to supply a large amount of raw materials to the reaction chamber. However, it is difficult to take out TBP of the saturated vapor pressure portion at a high flow rate by bubbling as described above.
Further, it is difficult to supply raw materials stably due to instability of the takeoff quantity of the group V element by bubbling as described previously.
As a method of takeoff of a large amount of gas from a liquefied organic group V raw material, it has been proposed in the Patent Provisional Publication Number 2-255595 that gas containing a group V element is supplied directly to a reaction chamber by the vapor pressure of the organic group V raw material so as to grow a compound semiconductor film without performing bubbling.